This invention relates to radar-controlled guidance systems for missiles and more particularly to a system which electronically compensates such a guidance system for the effects of directional errors suffered by the microwave guidance signal in traversing a radome which covers the receiving antenna.
When an antenna is enclosed in a radome, the apparent line of sight, generally does not coincide with the true line of sight. The angle between the apparent and the true lines of sight is called the radome-error angle or boresight error rate BER. The radome-error, as defined above, is not a characteristic of the radome alone, but rather depends upon the complex electromagnetic interactions of the complete housing system including the radome and the antenna.
One of the more serious problems encountered in radar-controlled guidance systems, having a radome-covered antenna, has been the development of a satisfactory radome. Apart from certain strength and temperature requirements, the radome design is largely a compromise between aerodynamic and electromagnetic performance. A long, slender, pointed radome is optimum aerodynamically, but cannot readily be made to have good electromagnetic performance, that is, it has a relatively large radome-error. With a blunt radome, acceptable electromagnetic performance can be more readily achieved, but the high drag due to a blunt radome seriously reduces the aerodynamic performance of the missile.
This invention contemplates the introduction of an electronic compensating voltage into the radar-controlled guidance system at a suitable point to reduce or to eliminate the effects produced by radome-errors, which, in the absence of such compensation, would produce a serious guidance defect in the system.
Electrical distortion of plane waves passing through the dielectric material of missile or aircraft radomes results in non-linear and varying boresight errors. The sign of the distortion has stability ramifications for missile guidance. This boresight error rate (BER) must be compensated in order to provide improved system performance.
Positive boresight error rates will result in an increase system gain, driving the system into a limit cycle at the missile body natural frequency. Negative boresight error rates will result in a low frequency phugoid motion which will perturb the intercept. Depending on the intercept scenario and the magnitude of the boresight error rate, the missile system effectiveness can be greatly reduced.
If the sign and magnitude of the boresight error can be determined and compensated, the missile system will remain effective. This invention is a robust filter technique to both learn the boresight error slopes and to compensate for them in generating missile guidance signals.
In the past several solutions to measure and correct boresight error have been attempted. These solutions have involved:
1. Minimization of boresight error by tuning radome materials and construction to the system's operating frequency. While such systems are theoretically very good, in practice, many factors work against this technique. For example, in flight, temperature variations and radome ablation may detune the system, and the system is therefore constrained to operate in a very narrow frequency band.
2. Correction of boresight error has been attempted by factory measurement of the error, and the use of compensation tables to provide the correction. This factory compensation method is very popular, but it suffers most of the limitation of the tuning method. Additionally, if a wider operating frequency is desired, factory testing time (and therefore costs) rise quickly, as does the compensation memory. Additionally, factory compensation is performed when the missile radome is not operating in the pressure and temperature regimes which are authentic for the flight of the missile.
3. Another method used to correct the boresight error problem is biasing the system to positive sign errors to provide protection against phugoid behavior. This method introduces a positive bias into the system to bias away from the negative behavior (phugoid) in favor of the positive behavior (limit cycle at natural frequency). Missile system are more tolerant of positive boresight error than negative error since the limit cycle frequencies are usually high enough to prevent trajectory disturbances. However, this method fails when the scenario is sensitive to any mismatch to boresight error as it does not compensate for the error, but simply biases away from the more sensitive signal. In addition, the radome is still required to have minimal boresight errors as the bias itself will be destabilizing above certain boresight error rate values.
4. Another method involves the running of a bank of Kalman filters with different assumed boresight error rate values and attempting to match observed line of sight behaviors to estimated line of sight behaviors given the BER corruptions. This method required a number of filters and therefore considerable computer memory and throughput requirements. This method cannot explicitly distinguish in-plane from cross-plane error combinations which would make different filters have similar outputs, allowing for incorrect compensations to be selected.
5. Still another prior art method involved driving the system bias to high frequency oscillations and observing the induced target line of sight rate under body motion. This method is similar to prior method 3, above, but continues to positive bias the system to a preset value or until the system displays the positive BER instability ("limit cycling at the body natural frequency"). Driving the positive BER instability limit cycle, the system estimates the effective BER and corrects the compensation. The weakness of this method is that the instability is not designed to make the BER observable and the method does not easily distinguish in-plane and cross-plane compensation, again resulting in incorrect compensation.